All aluminum versus copper/aluminum coils for air conditioner?

I've worked in the HVAC trade and would like to share some up-to-date facts
about copper versus aluminum condensing units and evaporator coils.
First, let me separate all A/C coils with fins from coils with all-aluminum
spines known as 'Spine-Fin' made by Trane and sold by Trane and related co
mpanies, used in outdoor condensing units and heat pumps. I recommend again
st products that use the spine-fin design. This design is notorious for plu
gging up; loading up with cottonwood fluff, spider silk, leaf litter, insec
t debris etc. Regular visits from the service man are a necessity to burn t
he debris out of the coil with a torch since washing with the powerful spra
y cleaners normally employed in coil cleaning is ineffective on spine-fin c
oils.
Copper tube coils with aluminum fins versus all-aluminum tube coils with al
uminum fins...
1a) Money - Copper prices went out of control high when the Chinese economy
boomed prior to the Great Recession of 2008. It only reasonable for U.S. m
anufacturers to anticipate future copper metal price spikes in considering
potential future cost and potential copper shortages. Aluminum metal is ple
ntiful, but uses a great deal of electrical energy to refine the ore.
Advantage aluminum.
1b) Aside from the cost of copper metal, aluminum coils are more difficult
and costly to manufacture. This is in large measure because it is difficult
to join aluminum tubes because of the thin layer of oxide that forms on al
uminum. Copper is much easier to join and repair. New aluminum soldering al
loys were developed over recent years that have improved the process of pro
ducing all-aluminum coils, although it is still remains difficult to repair
an aluminum coil in the field.
Advantage copper.
2) Higher refrigerant pressures - New EPA mandated chlorine-free refrigeran
ts replaced long time standard HCFC refrigerant R-22 in 2012. An example of
a new refrigerant is Carrier's Puron (which is known in the trade as R-410
a) which operates at higher pressure than R-22, so the tubing used in makin
g new condensing and evaporator coils must have thicker walls to resist the
higher pressure.
The new refrigerants heat-carrying ability, known as its enthalpy, fluid ou
nce for ounce, is less than that of R-22. Combined with other physical prop
erties, greater mass flow is needed to move the same quantity of heat, thus
larger coils are needed. Larger coils mean more metal used. A-coils in eva
porators (at the furnace end) become N-coils to absorb the heat from the ai
r stream, and condensers (found outdoors to move the heat into the air) mus
t be much larger to handle double the flow of refrigerant. EPA also mandate
d higher Seasonal Energy Efficiency Ratios (SEER) for air conditioners and
heat pumps, requiring these coils to be made larger still.
Advantage aluminum.
4) Corrosion - Copper coils are made with aluminum fins mounted in galvaniz
ed steel frames, which is a recipe for galvanic corrosion over time. Alumin
um metal reacts with the oxygen in the air to produce a thin, but tough oxi
de coating that protects the underlying metal from corrosion. If the alumin
um metal is scratched or scuffed, the protective oxide layer reforms almost
immediately. Aluminum coils are all aluminum, and being homogeneous, do no
t suffer from galvanic corrosion the way three-metal copper coils do. But t
he protection afforded by the oxide layer is not absolute; Aluminum is very
sensitive to chloride corrosion from animal urine, like dogs sent marking
or male cats spraying, sea salt spray, etc. While chloride will destroy bot
h copper and aluminum coils, chloride attacks aluminum coils much more aggr
essively.
Advantage aluminum, except by the ocean.
4a) Chinese Drywall, More Corrosion - References are made in this thread to
Formicary Corrosion a.k.a. Ant Nest Corrosion. Formic acid is an organic a
cid produced by ants to signal each other, but many organic acids can cause
this deep pitting corrosion in copper. Some thread entries blame ants, but
not one entry has named the actual cause behind most of the recent spate o
f damage to U.S. HVAC equipment - hydrogen sulfide off-gassed by impurities
in imported Chinese drywall. Because of its protective oxide layer, alumin
um does not react with hydrogen sulfide the way copper does.
Advantage aluminum.
5) Field serviceability - Repairing aluminum refrigeration tubing is much m
ore challenging than repairing copper tubing. Copper can be heated all the
way to cherry red glow without damage. It can be both brazed (temperature o
ver 800 degrees F) or soldered (temperature below 800 degrees F). Aluminum
heats up and as the temperature rises, without warning, melts onto a puddle
. When the temperature is tightly controlled, because of that thin, but ten
acious layer of aluminum oxide mentioned earlier, solder or braze filler me
tal alloys have difficulty bonding to aluminum. Recently developed exotic a
nd expensive fluxes are making field repair easier, but some technicians st
ill refuse to even attempt a field repair on an aluminum coil, insisting on
replacing it instead.
Advantage copper.
Conclusion.
I think it is fair to say the future is all aluminum coils, both because of
the cost savings in the less expensive metal, and continuing improvements
in aluminum joining and repair techniques.
Aluminum or copper, to stretch the life of your investment in A/C equipment
, consider this: if you would like your new A/C or heat pump to last as lon
g as possible, for a few hundred dollars extra, ask your contractor if the
brand and model they are offering is available with a 'Coastal' or 'Sea-Sid
e' option. Units with this option are made with their coils coated with a v
ery thin, but
completely corrosion resistant polymer; they are assembled using stainless
steel screws and bolts, and are painted with an extra durable paint. Coasta
l option units are intended for use near the bodies of salt water, but are
equally good at resisting corrosion caused by a range urban threats from ac
id raid to cat pee and many other chemical insults I can't name.
I hope all the above has proven useful at shedding light on the advantages
and disadvantages of aluminum and copper coils.

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